In Fig. 1 one can see that providing higher voltage pulses to the stepper motor pushes the motor curve upward and outward especially at higher speeds. The motor coils have inductance and resistance and the inductance resists changes in current. The motor coils have a time constant determined by their resistance and inductance, tau = L/R, with L (inductance) in Henrys and R (resistance) in Ohms. L/R has units of seconds. After this elapsed time, the coil has either charged or discharged 63% of its final value. At low pulse rates, the coil has enough time to charge up to the rated current. Since torque is proportional to current, if the rated current flows into the coil, the motor can produce the rated torque according to its torque vs. speed curve. However, as the pulse rate increases, the coil doesn’t have sufficient time to charge to its rated value. If the coil cannot charge to its rated value, the torque will be reduced. Eventually, as the pulse rate is increased, the torque production reduces further and until eventually almost zero current can be injected into the windings and the torque production is essentially zero.
When higher voltage pulses are used to drive the motor, the current rise can be increased. Voltage acts as a pressure to force current into the windings at a faster rate. When the current rises at a steeper rate, it can achieve the rated current value in a shorter amount of time. So raising the voltage provides more current than lower voltage especially as the pulse frequency increases. Higher current at a high pulse rate translates to higher torque at high speed. There are a couple types of drives used to achieve higher speeds with stepper motors.
An L/R driver increases the driver voltage while keeping the current flowing to the motor at or below the rated current. If one increases the current above the rated current, the motor will get very hot as the heating power is I2R. Suppose there is a motor rated for 6 VDC, 1 A, and 6 Ω . If the motor is supplied with only the rated voltage of 6V it will have only minimal performance (6VDC/ 6Ω = 1 AMP). However, if an external 6 Ω external resistor is placed in series with the windings then the total resistance per phase increases to 12 Ω . With the 6VDC signal, the motor will only receive 0.5 amps. Therefore, with the 6 Ω external resistor, we can now increase the driver voltage to 12 VDC, since I=12V/12 Ω = 1 amp which is the rated current. Since the circuit inductance, L, is essentially the same and the circuit resistance, L, has doubled the circuit time constant L/R is decreased and current will rise faster in the windings. So the torque vs. speed curve will be pushed out at higher speeds. Further, suppose we increase the external resistance to 18 Ω . The total resistance is now 24 Ω and with a 12 VDC power supply, the motor will only get 0.5 amps. Therefore, we can increase the driver voltage to 24 VDC, since I=24 D/24 Ω = 1 amp which, again, is the rated current. Because the circuit time constant, L/R, is further decreased, the torque/speed curve will be pushed out even more. The L/R driver is effective in that it increases the high speed torque. It is also relatively simple to implement. However, it is inefficient as the putting current through the added resistance increases the power loss.
PWM (chopper) drives
In a PWM or chopper drive a high voltage is applied to get the voltage to rise up at a steeper rate. Once the drive senses that the current has reached a certain rated value, the voltage supply is switched off. When current level drops below some set point, the high voltage is turned on again. The high chopping current is turned off and on to maintain some rated current value. The higher the chopping frequency of the drive, the better the high speed performance of the motor. A chopping drive will produce an audible noise if the chopping frequency is in the audible range. Some drives are designed to chop in the 35 kHz to 50 kHz range, beyond the range of human hearing. Also, while high chopping voltages increase the high speed performance, they also increase motor heating due to eddy current losses in the motor. With high chopping voltages, the temperature rise in the motor may exceed the recommended value and the duty cycle may need to be decreased.
Sources of higher DC supply voltage
L/R drives or PWN (chopper) drives require external power supplies to provide the appropriate input (bus) DC voltage. Most drives just convert whatever DC bus voltage is provided into a train of pulses of approximately the same voltage. Often stepper motors require pulses of 40+ volt to achieve the high speed end of their torque vs. speed curves. Many control systems have 24VDC readily available but do not have a source of higher DC voltage without adding a special DC power supply. Fastech is one drive supplier that builds a DC voltage converter into their drives. The customer just supplies 24 VDC to the Fastech drive and the drive itself boosts the voltage to the appropriate level. A Fastech motor and Fastech drive running on a 24 VDC supply can provide higher torque than a conventional drive and motor running on a 24 VDC supply. See the example Fastech curve below in Fig. 2.